Method for controlling an artificial orthotic or prosthetic knee joint

ABSTRACT

The invention relates to a method for controlling an artificial orthotic or prosthetic knee joint, on which a lower leg component is arranged and which is assigned a resistance device having at least one actuator, by means of which the bending resistance is modified depending on sensor data that is determined during use of the orthotic or prosthetic knee joint by means of a sensor, wherein the absolute angle of the lower leg component is determined exclusively by means of at least one inertial sensor, the angle determined is compared with at least one threshold value, and the bending resistance is modified when the threshold value is reached.

The invention relates to a method for controlling an artificial orthoticor prosthetic knee joint, on which a lower leg component is arranged andwhich is provided with a resistance device having at least one actuator,by means of which the bending resistance is modified as a function ofsensor data which are determined by means of at least one sensor duringuse of the orthotic or prosthetic knee joint.

Prosthetic or orthotic knee joints replace or support the function of anatural knee joint. In order to achieve a maximally optimalfunctionality of the artificial knee joint, there are a multiplicity ofdesigns on the market, which influence the behavior of the knee jointsduring the standing phase and the swinging phase. Mechatronic kneejoints are known, in which the movement situations are detected by meansof a plurality of different sensors and, on the basis of the sensordata, a resistance device, by means of which the bending resistance orthe extension resistance is varied, is controlled. One basic problem isthat the great variety of the possible movement situations can beencompassed only with difficulty in simple rules. In order to controlactuators and brakes, therefore, so-called state machines are used,which are highly complex and represent many different activities.Disadvantages with this are the long development time and the use ofelaborate components.

EP 1 237 513 B1 relates to a supporting device which replaces theexistence or function of a limb and consists of at least two parts,connected to one another by an artificial joint, and a control device.The supporting device comprises a sensor, which detects an inclinationangle relative to a line of gravity of a part connected to the joint andis coupled to the control device. The control device is arranged in sucha way that it influences the joint on the basis of inclination angledata communicated by the sensor. In one configuration, the inclinationangle sensor is arranged as a prosthetic knee joint on a thigh tube; inorder to enhance the data situation, a second sensor may be arranged onthe lower leg.

DE 10 2008 027 639 A1 relates to an orthotic joint for supporting ananatomical knee joint, having an upper joint part and a lower joint partwhich are connected to one another in an articulated fashion. A lockingelement for automatically unblocking and blocking the orthotic joint inan arbitrary position is provided, as is an actuation element for thelocking element and a sensor means for detecting relevant informationfor the unblocking and blocking. An evaluation unit for evaluating theinformation acquired, and for forwarding this information to a controland/or regulating unit for the actuation element, is likewise provided.The sensor means comprises at least two sensors from the group:inclination sensors, rotation angle sensors, acceleration sensors orgyroscopes, for acquiring information describing the movement stateand/or resting state of a person. Two sensors of one type or one sensoreach of different types may be selected. All the sensors are arrangeddownward of the anatomical joint, in particular knee joint.

It is an object of the present invention to provide a method forcontrolling an artificial orthotic or prosthetic knee joint, with whicha reliable and comfortable walking behavior of the prosthesis ororthesis can be achieved in a simple and economical way.

According to the invention, this object is achieved by a method havingthe features of the main claim. Advantageous configurations andrefinements of the invention are disclosed in the dependent claims, thedescription and the figures.

According to the method for controlling an artificial orthotic orprosthetic knee joint, on which a lower leg component is arranged andwhich is provided with a resistance device having at least one actuator,by means of which the bending resistance is modified as a function ofsensor data which are determined by means of at least one sensor duringuse of the orthotic or prosthetic knee joint, the absolute angle of thelower leg component is determined exclusively by means of at least oneinertial sensor, the angle determined is compared with at least onethreshold value, and the bending resistance is modified, in particularreduced, when the threshold value is reached, in particular exceeded. Inthis way, sensor data which detect various measurement quantities, forexample a torque, a force and an angle, which then need to be processedrelatively elaborately, for a control algorithm can be obviated. Therestriction to the use of absolute angles, which are measured by meansof one or more inertial sensors, the absolute angle of the lower legcomponent being determined, is a simple and at the same timesurprisingly reliable way in which control of a resistance change in aresistance or damping device of an artificial orthotic or prostheticknee joint can be produced. In this case, furthermore, a plurality ofthreshold values are established, which trigger adaptation of thebending resistance when reached or exceeded or fallen below. In thisway, it is possible to achieve a damping curve which is adapted to thepatient and allows a smooth gait.

According to one refinement of the invention, an angular velocity of thelower leg component is calculated from the sensor data of the inertialsensors, and the bending resistance is reduced only when the angularvelocity is not equal to zero, i.e.

the lower leg component is moved in the extension or flexion direction.This ensures that a resistance change is carried out only duringwalking, as a function of the absolute angle of the lower leg component.The combination of an absolute angle with the angular velocity for thedetection of a swinging phase, and therefore establishment of the timewhen a bending resistance is reduced from standing phase damping toswinging phase damping, has been found to be reliable even in the caseof slow walking speeds. It is also possible to reliably determine otherphases of the movement during walking by the combination of an absoluteangle and angular velocity, so that the bending resistance or theextension resistance can be varied in a scope such that a smooth gait isobtained and the patient receives reliable and effective support.

According to one refinement of the invention, the absolute angle isdetermined exclusively by means of one or more inertial sensors, whichis or are fastened on the lower leg component or on an orthotic orprosthetic component fastened distally thereon. The distally arrangedorthotic or prosthetic components are, in particular, prosthetic feet orbraces or arch supports for a natural foot in the case of an orthesis.The sensor or the sensors are preferably fastened medially and/orlaterally on the respective lower leg component, in order to determinethe absolute angle of the lower leg component, i.e. its position withrespect to a fixed reference quantity, in particular the vertical.

The absolute angle may be determined by means of 2D or 3D magnetic fieldsensors, 2D or 3D acceleration sensors and/or 1D, 2D or 3D gyroscopes.When a plurality of sensors are used, the sensor data of a plurality ofinertial sensors may be merged together in order to be able to reliablyestablish the actual orientation of the lower leg component on the basisof a plurality of different sensor signals. Measurement inaccuracies orperturbations in the case of one sensor can then be compensated for bythe other sensor or sensors.

The threshold value of the absolute angle of the lower leg component maybe adjusted to the value which the lower leg component adopts at the endof the standing phase, so that particular physical properties of thepatient can be taken into account by the individual adjustment.Likewise, a preferred movement pattern may be taken into account byadaptation of the threshold value of the absolute angle individually tothe gait of the patient by the orthopedic technician responsible. Theend of the standing phase is to be understood as meaning the instantwithin a walking cycle at which the front of the foot just still touchesthe ground during the rolling of the foot, immediately before the frontof the foot loses contact with the ground and is lifted. The knee jointusually then bends further, so that flexion in the knee joint increasesthe distance of the foot from the ground.

The bending resistance may be switched between two fixed values so thatthere is a low bending resistance during the swinging phase and a highbending resistance during the standing phase, or in an emergency mode.Such an emergency situation may be activated in the event of falling orstumbling. An increased bending resistance may then be provided in orderto prevent unbraked folding of the knee. The configuration of theactuator is advantageously selected in such a way that triggering by theactuator in order to reduce the damping cannot take place when there isan applied flexion moment. This may, for example, be achieved bydimensioning the power of the actuator to be less than the power whichis necessary in order to overcome the counterpressure of a prostheticjoint loaded with a flexion moment. In this way, an actuation lock ofthe actuator is achieved by virtue of the technical power configurationor mechanical configuration, so that additional controls or sensors arenot necessary. If the prosthesis user is in a position with a foldedknee which is loaded, then, owing to the selected low power of theactuator, adjustment against the internal pressure of the system cannottake place, so that opening of the damping and spontaneous reduction ofthe damping can be avoided in potentially hazardous situations, even ifthe predetermined other parameters for varying the flexion damping arefulfilled.

According to one refinement of the invention, bending resistance ismodified, in particular increased, when the angular velocity has reacheda zero point and reversal of the movement direction of the lower legcomponent is determined. If the bending resistance is increased at theend of swinging phase flexion, then increased security againstunintended folding in the event of collision with an obstacle isprovided. If the patient stumbles, increasing the bending resistancemakes it possible for the patient to be supported by the leg providedwith the orthesis or prosthesis, without the knee joint folding. Ifreversal of the movement direction is determined at the end of theswinging phase, the bending resistance may be reduced in order to permitthe initial standing phase flexion. Reversal of the movement directionat the end of the swinging phase may be determined by the angularvelocity reaching a zero point and the absolute angle being reduced.

The angular acceleration of the lower leg component may also bedetermined from the inertial sensor data, provision being made for thebending resistance to be increased, or not reduced, when a thresholdvalue is exceeded. This is used in order to detect special situations,for example when stumbling. The angular acceleration is preferablydetermined when walking forward, so that the most frequent walkingsituations can be recorded and taken into account. Exceeding of a limitvalue by the angular acceleration can indicate that the smooth walkingmovement is interrupted, so that a damping value other than that of theswinging phase damping seems expedient. In general, the flexion dampingis then to be increased or left at a high value.

An exemplary embodiment of the invention will be explained in moredetail below with the aid of the figures, in which:

FIG. 1—shows a schematic representation of a prosthetic device;

FIG. 2—shows a flow chart of the control; and

FIG. 3—shows a lower leg angle diagram.

FIG. 1 provides a schematic representation of a prosthetic leg with aprosthesis shaft 1 for receiving an upper leg stump and for fixing theprosthetic leg to a patient. Arranged at the distal end of theprosthesis shaft 1, there is a prosthetic knee joint 2 which is equippedwith a resistance device 3, for example in the form of a hydraulicdamper or a wrap spring brake. At the distal end of the prosthetic kneejoint 2, a lower leg tube 4 and a prosthetic foot 5 are provided asfurther distal components. The functional elements: prosthesis shaft 1,prosthetic knee joint 2, lower leg tube 4 and prosthetic foot 5 areenclosed by a cosmetic covering 6 in order to impart a natural overallimpression as far as possible.

In the exemplary embodiment represented, an inertial sensor 7 as anangle pickup is arranged on the lower leg component, which consists ofthe distal part of the prosthetic knee joint 2, the lower leg tube 4 andthe prosthetic foot 5. The inertial sensor 7 may be formed as a magneticfield sensor, acceleration sensor or gyroscope. It is also possible fora plurality of inertial sensors 7 to be arranged on the lower legcomponent, for example in addition to the fitting at the distal part ofthe prosthetic knee joint 2 on the lower leg tube 4 or the prostheticfoot 5. The acceleration sensors and magnetic field sensors may beformed as 2D or 3D sensors, and in order to determine gyroscope data agyroscope may be formed as a 1D, 2D or 3D gyroscope. A plurality ofinertial sensors of the same type may be arranged on the lower legcomponent, likewise inertial sensors 7 of different types, for examplean acceleration sensor and a gyroscope, may be fixed on the lower legcomponent.

The inertial sensor 7, which is formed as an angle sensor, determinesthe angle value of the lower leg component relative to a center of massline 8, which extends through a center of gravity 9. The center ofgravity 9 corresponds to the center of mass of the body of the patient,and the angle α is determined between the center of mass line 8 and thelongitudinal extent of the lower leg component through the center ofmass 9 of the body in the extended position of the prosthetic knee joint2 at the end of the standing phase. The orientation of the lower legcomponent in FIG. 1 is defined by the connecting straight line 10 alongthe longitudinal extent of the lower leg tube 4 through the tilt axis ofthe prosthetic knee joint 2. In the position represented, the prostheticknee joint 2 is in the extension position at the end of the standingphase. The existing angle α of the lower leg component relative to thecenter of mass line 8 is stored as a threshold value. At this value,there is extension of the prosthetic knee joint 2 and of the hip joint,and therefore a step rear position of the leg, and it is safe for theuser to initiate the swinging phase. If a rolling process of theprosthetic foot 5 in the anterior direction is simultaneouslydetectable, which is manifested by an angular velocity α′>0 and theangle increase, there is likewise a step in the anterior direction. Onthe basis of these data, an actuator 31 of the resistance device 3 ofthe prosthetic knee joint 2 is activated in such a way that existingstanding phase damping is reduced and is kept low until the angularvelocity α′ has reached a zero point in the central swinging phase. Anangle range for the position of the lower leg component at the end ofthe swinging phase of a normal step may be stored on the basis ofempirical values. If a full step is not executed, i.e. the angle rangeis not reached, or a non-smooth variation in the angular velocity α′ isestablished, the resistance device 3 is influenced by means of theactuator 31 in such a way that there is an increased bending resistance.If the interrupted step is continued, a reduction of the flexionresistance may be carried out again in order to end the step.

FIG. 2 represents the functionality of the control as a diagram. Afterthe start of the control, the respective sensor value is determined in afirst step 20. A plurality of sensor values 7 ₁, 7 ₂, 7 ₃ may bemeasured by means of the inertial sensors 7. Besides the possibility ofdetermining three sensor values 7 ₁, 7 ₂, 7 ₃ of the lower legcomponent, for example by using three different inertial angle sensors 7such as a magnetic field sensor, an acceleration sensor and a gyroscope,it is also possible to determine a plurality of sensor values by using aplurality of inertial sensors 7 of the same type. In principle, it isalso possible to detect the sensor value with only one inertial sensor7.

The data detected by the inertial sensors 7 are merged in a furtherworking step 21, in order to compensate for inaccuracies and have a datasituation which is as complete as possible for the calculation of theabsolute angle α. If only one inertial sensor 7 is provided, the data donot need to be merged.

In a subsequent evaluation step 22, the sensor data 7 ₁, 7 ₂, 7 ₃ of theangles α of the lower leg component with respect to the line of gravity8 are calculated. It is likewise possible to calculate the angularvelocity α′ of the lower leg component in parallel therewith in afurther working step 23.

The angle α, calculated for example by a Kalman filter, with respect tothe line of gravity 8, is then compared in a further step 24 with athreshold value X which was established beforehand. As soon as theabsolute angle α is greater than the preset threshold value X, in acontrol situation in which the angular velocity α′ is not taken intoaccount, the actuator 31 may be activated in a further step 26, in sucha way that the damping device 3 adopts a reduced resistance forinitiation of the swinging phase. If the threshold value is not reached,in the alternative step 27 the actuator 31 is not correspondinglyactuated and the resistance of the resistance device 3 remainsunchanged.

If the angular velocity α′ is also calculated together with the angle αin step 23, and the angular velocity α′ is greater than zero, then awalking movement is detected in step 28. In a combining step 25, theangle α and the angular velocity α′ are coupled with one another, and ifboth threshold values are present or are exceeded, the actuator isactivated according to step 26, while if one of the two threshold valuesfor the angle α or the angular velocity α′ is absent, the actuator 31 isnot activated according to step 27.

FIG. 3 represents a diagram of the lower leg angle α as a function oftime. The individual positions during a stepping cycle are representedabove the diagram. The step begins with planting of the heel, theso-called heel strike, so that the lower leg angle α is initiallyslightly increased. In the course of continued stepping, the prostheticfoot is fully planted and after about 0.4 second the lower leg, or thelower leg component, reaches the vertical, so that the lower limb angleα is 0°. As a result of movement further forward, the lower leg, or thelower leg component, is tilted further so that the lower leg α isincreased in magnitude. The end of the standing phase is reached afterabout 0.8 second, and the front of the prosthetic foot leaves the groundso that swinging phase flexion occurs. At the end of the swinging phase,in the case of normal walking, the reversal point is reached at a lowerleg angle of 45° . This is the case at about 1.3 seconds. A movementreversal subsequently takes place, the lower leg angle α is reduced inmagnitude and approaches the vertical, until at about 1.7 seconds itreaches the vertical again and is then tilted further relative to thevertical, but this time in the positive angle range, by the foot beingextended forward. Over the profile of the lower leg angle α, a pluralityof threshold values may be established for this lower leg angle α. Ifthese threshold values are reached or exceeded, or fallen below,depending on the direction in which the threshold value is considered,the variation of the bending resistance may be carried out; inparticular, after a maximum bending angle is reached, the bendingresistance may be increased in order to provide security againstunintended and unbraked folding of the knee joint in the event ofstumbling. Simple and effective control of the damping of the resistancedevice can be carried out by means of the setting of the lower legrelative to the vertical.

It has been found that the simple algorithm described above can be usedwell in order to switch to and fro between standing phase damping andswinging phase damping. The control is particularly reliable when theangle value α is greater than a previously stored threshold value, whichis adjusted by operating a control knob on the prosthetic device whenthe patient is in backward movement and the angular velocity α′ isgreater than zero. If both conditions are satisfied, the resistancedevice 3 may be switched over from high standing phase damping to lowerswinging phase flexion damping by switching a hydraulic valve or byreleasing a lock of a brake device, for example.

The use exclusively of inertial sensors reduces the costs, since thetorque sensors based on strain gauges for alternative methods are veryexpensive. Furthermore, inertial angle sensors are free from wear.

Besides the first derivative of the angle signal for determining theangular velocity, it is also possible to use the second derivative ofthe angle signal, in order to determine the angular acceleration. Theacceleration signal may be used to detect falling, the angularacceleration exceeding a fixed limit value indicating that the smoothwalking movement is interrupted and a different damping value would bemore expedient, usually increasing the flexion resistance.

Furthermore, by observing the angle profiles, it is also possible todeduce the walking speed; in addition, not only may the resistancebehavior, and therefore the damping behavior during the swinging phase,be switched to and fro between two values in a binary fashion, but theswitching may be adjusted in very small time intervals for discreteangle values or for any angle value, or any sampling of the angle value.For example, hydraulic valves may be adjusted stepwise or brake devices,which may likewise be used in resistance devices, may be adjusted in anadapted fashion in order to produce a smooth gait.

Furthermore, on the basis of the angle profiles, the resistance devicemay be adjusted in such a way as to allow standing phase flexion in theevent of a heel strike with an extended joint, by reducing the flexionresistance. After this has happened, a progressive variation of thedamping may also be provided, so that, after reduction of standing phaseflexion after the heel strike, strongly increasing damping occurs, whichallows bending up to a particular angle value after the heel strike; inaddition, a further increase in the angle is prevented by adjustment ofthe resistance device.

The artificial knee joints may be used both as prostheses, as describedin the exemplary embodiment, and as ortheses. The resistance devices maybe configured as simple locks, complex hydraulics or wrap spring brakes.By taking the angular velocity α into account, it is also possible touse drives in order additionally to permit folding or extension. Forsecurity against undesired folding or reduction of the damping underload, i.e. in the case of an applied flexion moment, the actuator may beconfigured in terms of power so that the switching power to be appliedwhen a flexion moment occurs lies above the output power of theactuator, so that reduction of the damping and therefore abrupt foldingcannot occur.

1. A method for controlling an artificial orthotic or prosthetic kneejoint (2), on which a lower leg component (4, 5) is arranged and whichis provided with a resistance device (3) having at least one actuator(31), by means of which the bending resistance is modified as a functionof sensor data which are determined by means of a sensor (7) during useof the orthotic or prosthetic joint, characterized in that the absoluteangle of the lower leg component (4, 5) is determined exclusively bymeans of at least one inertial sensor (7), the angle determined iscompared with at least one threshold value, and the bending resistanceis modified when the threshold value is reached.
 2. The method asclaimed in claim 1, characterized in that an angular velocity of thelower leg component (4, 5) is calculated from the sensor data of the atleast one inertial sensor (7), and the bending resistance is reducedonly when the angular velocity is not equal to zero. 25
 3. The method asclaimed in claim 1 or 2, characterized in that the absolute angle isdetermined exclusively by means of one or more inertial sensors (7),which is or are fastened on the lower leg component (4, 5) or on anorthotic or prosthetic component fastened distally thereon.
 4. Themethod as claimed in one of the preceding claims, characterized in thatthe absolute angle is determined by means of 2D or 3D magnetic fieldsensors, 2D or 3D acceleration sensors and/or 1D, 2D or 3D gyroscopes.5. The method as claimed in one of the preceding claims, characterizedin that the sensor data of a plurality of inertial sensors (7) aremerged together.
 6. The method as claimed in one of the precedingclaims, characterized in that the threshold value of the absolute angleof the lower leg component (4, 5) is adjusted to the value which thelower leg component (4, 5) adopts at the end of the standing phase. 7.The method as claimed in one of the preceding claims, characterized inthat the bending resistance is switched between two fixed values.
 8. Themethod as claimed in one of the preceding claims, characterized in thatthe bending resistance is modified when the angular velocity has reacheda zero point and reversal of the movement direction of the lower legcomponent (4, 5) is determined.
 9. The method as claimed in one of thepreceding claims, characterized in that the angular acceleration of thelower leg component (4, 5) is determined, and the bending resistance isincreased, or not reduced, when a threshold value is exceeded.